This invention relates to improvements in and to an external fixation device used in the treatment of bone fractures, and more specifically to an improved unilateral or one-sided, telescopic, single frame fixation device which can provide dynamic axial motion.
The purpose of an external fixator is to hold together by means of pins, the fragments or segments of a fractured bone so that healing may progress while allowing the patients to retain. mobility of neighboring joints. To perform this task, the fixator must be sufficiently rigid or stiff to support the loads imposed on it without allowing excessive movement at the fracture site.
The use of external fixation devices is an established and recommended form of treatment in many forms of limb injury and many different fixation devices have been designed and are known to the applicants. While some of these are generally considered satisfactory, it is believed that most are unnecessarily complex and do not provide the degree of rigidity required for optimal bone healing. In an effort to increase rigidity, designers have resorted to multilateral or multiaxial units (which include more than one external fixation body) but such units are bulky and difficult to handle and are technically complicated. In addition their bulkiness renders access to the wound as well as soft tissue management difficult.
Another drawback associated with known external fixation devices has been their inability to permit the pin clamps-and their attachments to the main body of the fixation unit to be easily and quickly adjusted by the surgeon and the restricted range of movements which such component parts can undergo. This has rendered difficult the entry of the retainer pins and as well the ease with which any rotational misalignment of the fractured bone can be corrected. In addition, there are limitations on the extent to which the spatial orientation of each set of retainer pins relative to the main body of the fixation unit can be varied, such variation being required to enable the unit to be easily installed and to permit the body of the fixator to be positioned parallel to the long axis of the bone, regardless of where the bone fracture occurs whether in a wide or narrow part of the bone.
While certain prior art external fixation devices are designed to provide dynamic axial motion, a problem common to such devices is the high bending moments which occur in the retainer pins and therefore the main body of the fixation unit, during ambulation. Studies have shown that these high bending moments cause the telescoping parts of the unit to jam especially where sliding friction is relied upon and the telescoping parts are made of the same metal, or metals having a high coefficient of friction. These units may provide a consistent dynamic axial motion (i.e. axial telescoping motion) at the fracture site by the application of a load through the fixator pins by an externally applied motor (referred to as "active dynamization"), but they cannot generally be relied upon to provide consistent and reliable passive dynamization through ambulation.
It is important that the fixator be sufficiently stable so that dynamic axial motion is ensured through telescoping of the device and not by instability of the fixator. Repeated axial movements may cause the telescoping unit to loosen or jam with time, which in turn may alter drastically the rigidity of the fixator and the efficacy of dynamization. For example, a significant problem with certain prior art units is their rotatory instability. One solution to this problem was to use a key-lock system, i.e. where the inner and outer telescoping members are part circular and a key engages within a groove extending along the inner member. Enhanced rotatory stability can be provided by a tighter fitting key-lock, but this results in stiction of the contact areas of the key-lock. Loose-fitting key-locks, where a keying pin engages in a key-way formed in the inner member, prevent this complication but cannot provide adequate rotatory stability.
Fixators have also been developed relying on rolling friction between the telescoping parts rather than sliding friction, these utilizing roller bearings rolling in ball bearing races. However, due to the fact that micromotion required for external fixators is in the order of 1 mm or less, such ball or roller bearings providing this motion will not roll but rather rock and will therefore brinell. Such brinelling is a cause of instability and/or jamming, which should be avoided in dynamizing external fixators.
A still further deficiency of some known unilateral, telescopic external fixators is their inability to vary or alter the rigidity of the device to axial motion of the telescoping parts during the course of fracture healing. It has been found that greater rigidity is required in the early phases of fracture healing and less rigidity later. It is desirable that means be provided so as to prevent passive motion during early stages of fracture healing of the telescoping parts of the fixator device until a set or predetermined load is exceeded.